1. Field of the Invention
The present invention relates to a light emitting diode (LED) head having a function of detecting positional displacement of a photosensitive member and an electrophotographic image forming apparatus employing the same, and more particularly to an LED head having a function of detecting positional displacement of a photosensitive member and an electrophotographic image forming apparatus employing the same which are suitable for use with an electrophotographic color printing apparatus.
2. Description of the Related Art
Conventionally, an LED head having a function of detecting positional displacement of a photosensitive member of the type mentioned employs, as disclosed in, for example, in Japanese Patent Laid-Open No. 9975/1992, a writing light source in the form of an array such as an LED array which is used also as a light source for detection of the position of a photosensitive member and uses a photodiode as a position detector for the photosensitive member.
FIG. 2 schematically shows a construction of such a conventional LED head as described above. Referring to FIG. 2, the LED head 1 shown includes a position detecting LED array 4 for emitting light to position detector 3, a latent image forming LED array 5 for irradiating light upon a photosensitive belt 2, and a single rod lens array 8. Therefore, photosensitive belt 2 and position detector 3 must be disposed within a range of a focal length of the rod lens array 8. Usually, the range of the focal length is 0.2 mm or less. Actually, since the photosensitive belt 2 has some thickness, a gap 9 between the photosensitive belt 2 and the position detector 3 is smaller than 0.2 mm. The photosensitive belt 2 and the position detector 3 disposed with such a small gap left therebetween are brought into contact with each other readily, for example, by thermal expansion of them by a temperature variation or an impact upon the apparatus by an operator. Such contact between them gives rise to formation of a defective image because toner powder on the photosensitive belt 2 is scattered to damage an image or the speed of movement of the photosensitive belt 2 is dropped to cause partial contraction of an image.
Thus, the conventional LED head having such a function of detecting positional displacement of a photosensitive member as described above is disadvantageous in that, as the photosensitive member and the position detector are brought into contact with each other by thermal expansion of the apparatus or by some other cause, a defective image is produced because toner powder on the photosensitive member is scattered to damage an image or the speed of movement of the photosensitive member is dropped to cause partial contraction of an image.
The reason is that, because the rod lens array of the LED head has an equal focal length between a portion thereof which is used for detection of the position of the photosensitive member and another portion thereof which is used to write a latent image and besides the range of the focal length of the rod lens array is narrow, the photosensitive member and the position detector must be disposed within the narrow range of the focal length.